In oil drilling operations, a drilling fluid is typically circulated downwardly through a drill string to cool and lubricate the drill string, suspend the cuttings removed from the well bore, and to keep out formation fluids.
Both aqueous and oil-based drilling fluid systems are known. The more economical aqueous systems are used when practicable with oil-based systems being used where increased lubricity at the drilling head is desirable or when traversing formations which would be adversely affected by a water-based system, such as water soluble shale formations.
Aqueous drilling fluids typically comprise a viscosifying agent, generally a clay (e.g., a solid phase bentonite, attapulgite, or sepiolite), and a water fluid vehicle. In addition, salt or salt water can be added to the components of the drilling fluid to prepare a salt water drilling fluid. Various additives are also commonly employed to control viscosity, yield point, gel strength (thixotropic properties), pH, fluid loss, tolerance to contaminants (e.g., salt and calcium carbonate), lubricating properties, filter caking properties, cooling and heat transfer properties, and tolerance to inactive solids such as sand and silt or active native mud making clays (e.g., smectites, illites, kaolinites, chlorites, etc.). Clays are not usually used as the sole viscosifying agent, and typically organic water-soluble polymers (e.g., starch, carboxymethylcellulose, natural gums, or synthetic resins) are used in conjunction with clays. These polymers also aid the clay component of the drilling fluid to serve as a filtration aid to prevent or retard the drilling fluid from being lost into the formation.
Some well operators have used hollow microspheres (also referred to as “bubbles”) to reduce the density of the drilling fluid (mud). Certain combinations of aqueous drilling fluids (e.g., those comprising polysaccharides and dissolved salt (e.g., dissolved NaCl and CaCl2)) and certain glass bubbles (e.g., soda-lime-borosilicate glasses such as those marketed by 3M Company, St. Paul, Minn., under the trade designation “HGS18000”) undesirably tend to form a gel, rather than remain as a liquid. There is a need to provide such combinations of materials together that do not gel (i.e., remain fluid).